Revision: Atoms and Molecules Science SSLC (English Medium) Class 10 Tamil Nadu Board of Secondary Education

Metal:  A chemical element that is an effective conductor of electricity and heat can be defined as a metal.

Ex.: Copper, Iron, Silver, etc.

Metalloid: Metalloid is a chemical element that exhibits some properties of metals and some of non-metals. Metalloids are generally semi-conductors.

Ex.: Silicon. Arsenic, Antimony and Boron.

An atom is the smallest particle of a chemical element that retains its chemical properties.

Chemical bond— A chemical bond is the binding force between two or more atoms of a molecule.

Element: It is a substance that cannot be broken down into simpler substance by chemical means

Ex.: Oxygen, Hydrogen, Gold & Helium.

An atom is the smallest particle of an element which retains its chemical identity in all physical and chemical changes.

Radicals : A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.

An Atom: Smallest particle of an element that can exist and have properties of an element.

Relative atomic mass— Relative atomic mass is the mass of an atom of an element as a multiple of the standard atomic mass unit.

The relative atomic mass of an element is the ratio between the average mass of its isotopes to 1/12th part of the mass of a carbon – 12 atoms. It is denoted as A_r.

Relative atomic mass = `" Average mass of the isotopes of the element"/(1"/"12^{"th"}" of the mass of one Carbon- 12 atom")`

Compound: A compound is a pure substance that is formed when the atoms of two or more elements combine chemically in definite proportions.

Ex: H20, NaCl.

Non-Metal: Non-metal is an element that doesn’t have the characteristics of metal including, (i.e.) ability to conduct heat or electricity luster or flexibility.

Ex. Carbon Iodine, Sulphur.

Mass number— Mass number is the sum of the number of protons and neutrons present in the nucleus of an atom. It is denoted by A.

An atom which becomes charged by losing or gaining electrons is called an ion.

Atom: An atom is the smallest indivisible unit of an
OR
Atom is the smallest unit of matter.

Molecule : Molecule is the smallest unit of a compound (or an element) which always has an independent existance.

Covalent bond— When atoms of different non-metals neither donate nor accept electrons and hence no ions are formed, such a bond is called covalent bond.

Relative atomic mass is defined as the ratio of the average atomic mass to the unified atomic mass unit.

Relative atomic mass (A_r) = `"Average mass of the atom"/"Unified atomic mass"`

The mass of a single atom of an element is called the atomic mass.

Mass number : Numbr of protons and neutrons present in the nucleus of an atom.

Atomic number : Number of protons present in the nucleus of an atom.

Atomic number— Atomic number refers to the number of protons present in an atom. It is denoted by Z.

Atomic weight : It is the ratio that tells how many times an atom of an element is heavier than atom of Hydrogen

Mass number : Numbr of protons and neutrons present in the nucleus of an atom.

Atomic number— Atomic number refers to the number of protons present in an atom. It is denoted by Z.

Atomic number : Number of protons present in the nucleus of an atom.

Atomic weight : It is the ratio that tells how many times an atom of an element is heavier than atom of Hydrogen

Atomic weight : It is the ratio that tells how many times an atom of an element is heavier than atom of Hydrogen

Atomic number— Atomic number refers to the number of protons present in an atom. It is denoted by Z.

Mass number : Numbr of protons and neutrons present in the nucleus of an atom.

Atomic number : Number of protons present in the nucleus of an atom.

The weighted average of the masses of all its isotopes in a sample of that element is called the average molecular mass.

The sum of the atomic masses of all atoms in a molecule is called the molecular mass.

The relative molecular mass of a compound is the number that represents how many times one molecule of the substance is heavier than `1/12` of the mass of an atom of carbon ₆C¹².

A mole is the amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 grams of carbon -12.

One mole of any gaseous molecules occupies 22.4 dm³ (litre) or 22400 cm³ (ml) at standard temperature and pressure (STP). This volume is known as the molar volume.

Mole is the amount of a substance containing elementary particles like atoms, molecules or ions in 12 g of carbon - 12.

"The relative atomic mass or atomic weight of an element is the number of times one atom of the element is heavier than `1/12` times of the mass of an atom of carbon - 12".
Relative atomic mass = Mass of 1 atom of the element `1/12` of the mass of one C¹² atom.

Avogadro’s number is defined as the number of atoms present in 12g of ₆C¹² isotope i.e. 6.023 × 10²³ atoms.

The quantity of the element which weighs equal to its gram atomic mass is called one gram atom of that element.

Avogadro's number is defined as the number of atoms present in 12 g (gram atomic mass) of C-12 isotope, i.e., 6·022 x 1023 atoms.

OR

Avogadro's number is the number of elementary units, i.e., atoms, ions or molecules present in one mole of a substance. It is denoted by N_A.

A formula that gives the simplest whole number ratio of atoms of each element in a compound.

An atom is the smallest particle of an element that can take part in a chemical reaction; however, it may or may not exist independently. 

A molecule is the smallest particle of an element or a compound that can exist by itself; it never breaks up except for taking part in a chemical reaction.

The ratio between the masses of equal volumes of gas (or vapour) and hydrogen under the same conditions of temperature and pressure.

Vapour density is defined as the ratio between the masses of equal volumes of gas (or vapour) and hydrogen under the same conditions of temperature and pressure.

The molar volume of a gas is the volume occupied by one gram molecular mass or simply, by one mole of the gas at S.T.P. It is equal to 22·4 dm³.

The average atomic mass accounts for the different isotopes of an element and their natural abundances.

\[M_{\mathrm{avg.}}=\frac{M_{1}\times r_{1}+M_{2}\times r_{2}+M_{3}\times r_{3}}{r_{1}+r_{2}+r_{3}}\]

where M₁, M₂, M₃ are atomic masses of isotopes and r₁, r₂, r₃ are their relative abundances.

Mass % of element\[=\frac{\text{Mass of element in compound}}{\text{Molar mass of compound}}\times100\]

Shows the exact number of atoms of each element present in one molecule of a compound.

Molecular formula = (Empirical formula)_n​

\[n=\frac{\text{Molecular mass}}{\text{Empirical formula mass}}\]

\[x_A=\frac{n_A}{n_A+n_B}\quad x_B=\frac{n_B}{n_A+n_B}\]

x_A​ + x_B ​= 1

One mole of a gas = 22·4 litres at S.T.P.

The relative molecular mass of a gas or vapour is twice its vapour density.

2 × Rel. V.D. = Rel. molecular mass of a gas or vapour.

According to the given information, water has been taken from different sources and still decomposes into the same amount of oxygen and hydrogen; therefore, this is an example of the law of constant proportions.

"Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules."

• Proposed by Avogadro in 1811.
• 1 mole of any gas at STP = 22.4 L (at 0°C, 1 atm) or 22.71 L (at 0°C, 1 bar — new IUPAC STP).
• 1 mole of any substance = 6.022 × 10²³ particles.

Avogadro's Law (Volume–Moles Relationship):

At constant temperature (T) and pressure (P), volume is directly proportional to number of moles.